As a conventional technique relating to an electron tube, for example, there are known the techniques described in Japanese Unexamined Patent Publication No. 2013-19719, U.S. Pat. No. 5,500,531, and Japanese Patent No. 3470077. Japanese Unexamined Patent Publication No. 2013-19719 describes a flame sensor including a lower lid provided with a cavity by etching, an upper lid joined to the lower lid so as to close the cavity, and an electrode disposed in the cavity. U.S. Pat. No. 5,500,531 describes a discharge type sensor for detecting ultra-violet rays, including a silicon substrate having a cavity formed by etching, a glass substrate provided on the silicon substrate, and an electrode provided in the cavity.
Japanese Patent No. 3470077 describes a discharge light-emitting device including a substrate and a transparent substrate that are stacked on each other, and an internal electrode and an external electrode that is formed on the substrate and the transparent substrate. In the discharge light-emitting device described in Japanese Patent No. 3470077, a discharging space is formed between the substrate and the transparent substrate.